Silverlight Error "Layout Cycle Detected Layout could not complete" when using custom control

Question

I'm building a custom control in Silverlight by deriving from ContentControl and doing some special formatting to put a dropshadow behind the contents.

I've nearly got it working but have recently ran into a bizarre error. It works fine if it contains anything besides a Border, or a Grid/Stackpanel/etc that does not have an explicitly defined height and width.

I get a JavaScript error in IE, and the text says:

Runtime Error 4008... Layout Cycle Detected... Layout Could Not Complete.

If I specify a height and width on the contained grid/stackpanel/etc it works fine.

There is a ton on the web about this error when too many textboxes are used (over 250), but I'm able to reproduce my error with a single button in a grid.

I have no textboxes at all on the page. The error has to do with a detected infinite loop. I set a few breakpoints in the code and it seems that the "SizeChanged" event is getting called a lot during rendering, and each time the height/width increments by 10.

I'm assuming that setting a default height/width causes it to skip this incrementing of the number, but I have no idea why this error is happening.

Has anyone ran into this or have any ideas?

Answer 1

There is a good blog post on this error here.

Bascially what can happen is your changing some size in a measureoverride somewhere which causes another measure which changes the size which causes a measure and so on. I ran into this once before and fixed it by removing any code that caused a layout update or triggered a layout update during the layout cycle.

Answer 2

A common cause is handling SizeChanged and then in the handler doing something that affects the size of the element. Sometimes this is not obvious - it could be modifying child elements which affect the size of their container for instance.